1. Field of the Invention
The present invention relates to an image heating apparatus, adapted for use as heat fixing device to be incorporated in an image forming apparatus such as a copying apparatus or a printer.
2. Description of the Related Art
As heat fixing device for an unfixed image for use in a copying apparatus or a laser beam printer, there has been proposed and commercialized a heating apparatus of film heating type, as proposed in Japanese Patent Application Laid-open Nos. S63-313182 and H04-044075.
In such film heating type device, a heat-resistant thin film (fixing film), constituting a rotary heating member, is contacted, by means of a pressurizing elastic rotary member (pressure roller), with a heating member and is conveyed in a sliding motion thereon. A material to be heated, namely a recording material bearing an unfixed image, is introduced into a pressure nip portion, formed by the heating member and the pressure roller through the fixing film, and is conveyed together with the fixing film. Thus, the unfixed image on the recording material is fixed, by the heat supplied from the heating member through the fixing film and the pressure in the pressure nip portion, as a permanent image on the recording material.
FIG. 18 is a schematic view showing the structure of a prior heat fixing device of the film heating type, having a maximum sheet passing width corresponding to A3 size. Also FIG. 19 is a perspective view of the heat fixing device shown in FIG. 18, in which the heating film is omitted.
The heat fixing device 10 is provided with a film unit 1, and a pressure roller 4 which forms a pressurized nip N with the film unit 1. The film unit 1 is principally provided with a heating member 3, a cylindrical film (fixing film) 2 driven in a rotary motion in contact with the heating member 3, and a film guide 5 serving as a support member for the film 2 and the heating member 3. The film unit 1 further includes a pressurizing stay 6 which presses the film guide 5 toward the pressure roller 4.
The heating member 3 is supported by pinching between the film guide 5 and the film 2. The fixing film 2 is formed by a heat-resistant film of a circular cross section, loosely fitted outside the film guide 5.
More specifically, the heating member 3 is formed by a plate-shaped ceramic base material of a low heat capacity, such as of alumina (Al2O3) or aluminum nitride (AlN). On a surface of such base material, a heat-generating pattern, formed for example by silver-palladium (Ag/Pd) or Ta2N, and current-supplying electrode patterns of a low-resistance material, such as Ag, for supplying the heat-generating pattern with a current, are formed for example by screen printing. Also the surface bearing the heat-generating pattern is covered by a thin glass protective layer.
The pressure roller 4 forms a pressurized nip portion N in cooperation with the heating member 3 and through the film 2, and serves as outer film surface contact-drive means which drives the film 2 in rotation. The pressure roller 4 includes a metal core 4a, an elastic layer 4b formed for example by silicone rubber, and an outermost releasing layer 4c. The pressure roller 4 is pressed, under a predetermined pressure by unillustrated bearing/urging means, to the surface of the heating member 3, through the film 2. The pressure roller 4 is driven in rotation by an unillustrated motor, thus providing the film 2 with a conveying power by a friction between the pressure roller 4 and the external surface of the film 2.
As shown in FIG. 19, fixation flanges 8 are provided on longitudinal ends of the pressurizing stay 6. The fixation flanges serve to apply a pressure of an unillustrated pressurizing spring in a direction B, thereby transmitting the spring force to the pressurized nip portion N through the pressurizing stay 6, and to restrict the motion of the ends of the film 2 in the longitudinal direction thereof.
Now the functions of the heat fixing device 10 of the above-described structure will be explained.
Referring to FIG. 18, the heating member 3 generates heat by a current supply in the heat-generating pattern through the current-supplying electrode patterns, whereby the entire heating member shows a rapid temperature increase. The temperature increase in the heating member 3 is detected, as shown in FIG. 18, by a thermistor 7 provided as temperature detection means in contact with or in the proximity of the heating member 3, and is fed back to a current-supply control part 70 for the heating member. The current-supply control part 70 controls the current supply to the heat-generating pattern in such a manner that the temperature of the heating member, detected by the thermistor 7, is maintained at a substantially constant predetermined temperature (fixing temperature). Thus the heating member 3 is controlled at a predetermined fixing temperature.
Thus, there is reached a state where the heating member 3 has a predetermined temperature and the film 2, rotated by the pressure roller 4, has a constant peripheral rotation speed. In such state, a recording material P, bearing an unfixed image T, is conveyed from a transfer part of an unillustrated image forming apparatus and is introduced into the pressurized nip portion N formed by the heating member 3 and the pressure roller 4. Thus, the recording material P is conveyed through the pressurized nip portion N together with the film 2. Therefore, the heat of the heating member 3 is given to the recording material P through the film 2, whereby the unfixed visualized image (toner image) T on the recording material P is heat fixed onto a surface thereof. The recording material P, after passing the pressurized nip portion N, is separated from the surface of the film 2 and is further conveyed. A prior fixing apparatus shown in FIG. 18 is a center-reference apparatus, in which the recording material is conveyed in such a manner that a longitudinal center of the apparatus coincides with a center in the width direction of the recording material P.
In the film heat fixing device 10 of such type, particularly when recording materials P, having a width smaller than the length of the film 2 in the generating line thereof (such recording material being hereinafter called “small-sized sheet”), are conveyed in succession, the pressure roller 4 shows a thermal expansion in sheet non-passing areas in the longitudinal direction thereof. As a result, the fixing film conveying speed of the pressure roller 4 becomes larger in the sheet non-passing areas than the fixing film conveying speed of the pressure roller 4 in the sheet-passing area, and the fixing film 2 shows a difference in the rotation speed, along the longitudinal direction thereof, between the sheet non-passing areas and the sheet-passing area. As a result, there is generated a phenomenon that the fixing film 2, within an area thereof immediately before entering the nip portion N, becomes slack in a longitudinal central area (sheet-passing area). It is also already known that such slack in the film tends to generate creases in the paper. In order to prevent such creases in paper, a fixing apparatus is proposed as disclosed in Japanese Patent Application Laid-open No. H10-247026.
FIG. 20 is a schematic perspective view of a fixing apparatus disclosed in Japanese Patent Application Laid-open No. H10-247026, seen from an upstream side of the conveyance direction, and omitting the fixing film.
In such fixing apparatus 10, protruding portions 11 are formed on the both ends in the longitudinal direction of a film guide 5, in a lateral face at the upstream side in the conveyance direction, and a protruding portion 12 is formed in a central part in the longitudinal direction of a pressurizing stay 6, on an upper face thereof.
FIGS. 21 and 22 are schematic views respectively showing through section in a central part in the longitudinal direction, and through section in an end part in the longitudinal direction, when the recording material P is conveyed in such fixing apparatus 10.
Separately from the pressurized nip N formed by the film 2 and the pressure roller 4, a contact area of the film 2 and the recording material P is defined as a contact nip M. Also a line, formed by connecting, along the longitudinal direction of the fixing apparatus, upstream ends of the contact nip M in the conveyance direction of the recording material, namely positions at which an arbitrary point on the recording material P starts to contact with the film 2, is defined as an entrance tangential line Ma. Also a line, formed by connecting, along the longitudinal direction of the fixing apparatus, downstream ends of the contact nip M in the conveyance direction of the recording material, namely positions at which an arbitrary point on the recording material P starts to be separated from the film 2, is defined as a separating tangential line Mb.
Referring to FIGS. 21 and 22, the cross sectional shape of the film 2 under running expands, at end portions in the longitudinal direction, in a direction K (toward upstream side in the conveyance of the recording material P) because of the protruding portions 11, and, at the central part of the longitudinal direction, in a direction J because of the protruding portion 12. By these functions, the pressurized nip N, the contact nip M and the running state of the film 2 assume a relationship as shown in a plan view in FIG. 23.
More specifically, in the proximity of the end portions in the longitudinal direction, the film surface extends farther in a direction toward the upstream side in the conveyance direction, and, in the proximity of the center in the longitudinal direction, the film surface becomes concave toward the pressurized nip N. Therefore, the entrance tangential line Ma assumes an approximately arc shape, whereby the recording material P tends to be stretched toward the outer sides in the longitudinal direction. This function suppresses generation of paper creases.
The fixing apparatus of this type is effective significantly against the paper creases, but may result in a drawback, particularly in case of passing a wide-sized paper (recording material of a width close to the film length along the generating line), of a decrease in the image density in the end portions of the recording material in the width direction thereof. Such drawback is induced by following reasons.
In the course of fixation conveyance of a wide-sized paper, the both end portions of the recording material tend to be curled upward (toward the film) by an outward tension on the recording material P by the paper creases preventing function, and the both end portions of the entrance tangential line Ma in the longitudinal direction are more stretched, than the proximity of the central portion, from the pressurized nip N toward the upstream side. Therefore, the unfixed image in both end portions comes into contact with the film, considerably in front of the pressurized nip N. The image may be easily perturbed because of these facts.
In consideration of such situation, a fixing apparatus is proposed as disclosed in Japanese Patent Application Laid-open No. 2001-183930. FIGS. 24 to 26 illustrate an example of such fixing apparatus 10, wherein FIG. 24 is a perspective view seen from the upstream side in the conveyance direction of the recording material P; FIG. 25 is a perspective view seen from the downstream side in the conveyance direction; and FIG. 26 is a plan view.
In such fixing apparatus 10, as shown in FIG. 24, protruding portions 11 are formed in areas inside by a certain amount from the both ends in the longitudinal direction of a film guide 5, at an entrance side therefore for the recording material, and a protruding portion 12 is formed in a central part in the longitudinal direction of a pressurizing stay 6, on an upper face thereof. Also, as shown in FIG. 25, a protruding portion 13 is formed in a central part in the longitudinal direction of the film guide 5 at the exit side of the recording material, and protruding parts 14 are formed on the fixation flanges 8 at the exit side of the recording material.
In case of conveying the recording material P by this fixing apparatus 10, the pressurized nip N, the contact nip M and the running shape of the film assume a relationship as shown in FIG. 26.
The fixing apparatus disclosed in Japanese Patent Application No. 2001-183930 has such a running state that the end portions of the entrance tangential line Ma, in the longitudinal direction thereof, do not protrude toward the upstream side, in comparison with the fixing apparatus disclosed in Japanese Patent Application No. 10-247026. In case of utilizing the fixing apparatus of Japanese Patent Application No. 2001-183930, the paper crease preventing function somewhat decreases in comparison with the fixing apparatus of Japanese Patent Application No. 10-247026, but the aforementioned density loss in the end portions is improved.
Also Japanese Patent Application No. 2001-185328 proposes a film unit structure, effective against the density loss in the end portions, by forming protruding portions in various parts of a film guide member, a pressurizing stay and fixation flanges.
However, even in such fixing apparatus, it is difficult to improve both the paper creases and the density loss (image perturbation) in the end portions, and there is observed a apparently trade-off relationship that an improvement in either property results in a deterioration in the other property.
Also in fixing apparatuses disclosed in Japanese Patent Application Laid-open Nos. 2001-183930 and 2001-185328, protruding portions are provided not only on the lateral face of the film guide at the upstream side in the conveyance direction, but also on the lateral face at the downstream side in the conveyance direction and on the upper face of the pressurizing stay, thereby significantly increasing the tension of the film. Therefore, the original drivability of the film is worsened, and, for example in a thin paper left standing in an environment of a high temperature and a high humidity, there are often experienced defects in fixation conveyance, such as an image perturbation and a sheet jamming, caused by a film slippage.